December
December 4
WOOFER CONES, DANGEROUS TO FINGERS WHEN IN MOTION !
As absurd as it sounds, I nearly broke an index finger on Thursday night. I got the "bright" idea to touch one of the midbass driver cones while it was in operation (an action I have done in the past with earlier 18" drivers with no ill effect). I was playing a Barry White tune from 1973, so the bass frequencies were pretty high, but the SPLs were in the 140dB range, so the driver was making close to 1/4" of excursion. And when my finger met up with it, it got dislocated. I didn't expect the amount of force and thought there would be sufficient elasticity in the cone to present no danger. WRONG! My finger will recover, but I feel like an idiot for not connecting the physics of modern driver technology with the awareness of velocity and kinetic energy and the logical deduction that something traveling at over 600mph, even if only for a 1/4" distance, is going to produce a severe impact. As I type this, the pain in my finger is a constant reminder of that fact.
ARCHIVING MUSICAL MEMORIES
I have been putting off a project for a long time, but this month have finally made some concrete steps toward moving my rather massive library of recorded open reel tapes and cassette tapes to digital format. For a couple of years, I was put off by the sheer scope and volume of the project. But this month I decided to "just do it." That meant going through my directory book where all my reels are catalogued and determining which tapes are Dolby and which are dbx Type 1. Most of the tapes after summer 1984 were the latter.
The next step was to mark the catalogue entries as to the kind of noise reduction method used and then mark the tape labels. I have them numbered consecutively on the shelves.
Finally, I dragged the Akai GX-630DB over to where the audio workstation is and connected it up to two available input channels on the MotU 896. After a brief gain calibration process, I was ready to start recording. Since I have a lot of classical LPs on these reels that I took out from the library in the 1980s, I chose to start with the bunch of these. They needed quite a bit of cleanup. My tool of choice was SoundForge and two Steinburg plugs, DeNoiser and DeClicker. I have to say that they did a marvelous job of removing the snap, crackle, pop from the worst of the recordings, without producing much in the way of objectionable artifacts.
Much of the tape is Ampex 641. A few of the reels are Scotch 212 and 207. I have one reel of Scotch Master, which, back in the 1970s, cost me over $35 for that one reel. I used that for critical live recordings. Live music, fireworks, aircraft takeoffs, etc.
Next, I started working on my stable of live recordings that I did in the early 1980s. I used to work FoH as a sound mixer for a fifties band. Those guys could sure play long sets. Some of them were 80 minutes of high energy playing, without a break. One reel of 3600' of Ampex 641, nearly full, both sides. Close to 3 hours of continuous oldies. I spent some time sweetening the recording. Those guys were always bass-shy because they didn't have a good bass head/speaker combo and not enough juice to mic the kick drum, so there was no thump to their sound at all. One of the first things I did with the recording, one in the digital domain, was to strike a balance between reality and a more desirable sound, by emphasizing what little weak bass existed, plus sweetening the highs a bit, since the bias on these decks was normally too high for Ampex 641 and resulted in a modest and gradual rolloff of highs above 6KHz. I'd say I made good progress. The final recording sounded pretty listenable. And it was fun hear the band play through a real sound system.
Each reel had unique material, and totally different recording conditions, so each reel required a different set of decisions about EQ, noise reduction and so on. Some reels suffer the dreaded "right channel bass crosstalk" from the material on the reverse side. That was one thing that always irked me about open reel 1/4 track tape. That awful crosstalk. Why couldn't the designers allocate tracks so that the crosstalk would be between left/right where it wouldn't be noticed, instead of program from the reverse side bleeding through to the right channel where it would be objectionable? Probably to make the L/R crosstalk specs look good, I suppose. Cassettes don't seem to suffer this problem. But all through my years, dating from the first 1/4-track stereo reel deck to the state of the art Akai GX747dbx, the problem was always inherent in the format. One solution was to simply not record the second side, but then you may as well use a half track machine like the Otari MX5050BII. But you know what? The dang tapes were expensive! Heck, when I had my Karg Laboratories FM tuner, which, without the stereo adapter, was just a nice monophonic tuner, I used to record separate programs on right and left channels! I'd use all four tracks, but get 12 hours of program onto a single reel of tape. Wow, those were the days. I'm getting goose bumps just thinking about the memories!
Then there are some really ancient tapes. I have stuff going back to post WWII Germany, which was produced before acetate backed tape was invented, so the backing is paper. Paper recording tape has got to be the most delicate tape ever made. This is the stuff you play through once, because the oxide shedding is enormous. There is no lubricant coating on any of it. But there it is.. I have some Deutchmusik on reels of tape that are over 60 years old and the play beautifully on the Akai GX747dbx, which is gentle enough stopping and starting so as not to break the tape. The tapes do have some splices in them that were made God-knows-when, so their adhesive was completely dried up and so when the tape reaches a splice, the deck would shut off because the tape fell apart at those locations, but a little Scotch magic tape on the splicing block and things were back in business. They are a challenge to clean up though, because the high frequency loss starts at about 200 Hz and drops about 6dB/octave from there. Since 36dB of boost at 6400Hz is needed to restore the original tonal balance, at least 40dB of noise reduction is needed to compensate and also knock down some of the tape hiss. Every reel is a special and different challenge. And, oh the memories!
EXTERNAL PROCESSOR LOOP IMPROVEMENTS
While making some infrasonic test measurements the other day, I became curious as to the vast difference in system out put below 10Hz when the external processor loop is switched in and out of the system. I could even hear an audible change in the timbre of a 32' organ pedal stop, with and without the external processing chain engaged.
This discovery prompted me to take, one by one, the equalizers and dynamic processors from the chain and put them on the test bench and measure their frequency response. Half of the devices could not pass a 50Hz square wave without considerable tilt! The worst offender of the bunch was the dbx 4bx. So I set to work trying to unravel the mysteries of that beast. The response was down almost 3dB at 16Hz, an absolutely unacceptable level of performance.
As it turns out, there are three banks of VCAs in there, all custom chips with the dbx logo on them, one for each frequency band. The high band had something like a .047uF capacitor coupling its output to the summing circuit. The midrange VCA had a 1 uF non-polarized capacitor. Now one would think that the low band VCA would have something much larger, but that was not the case--it also had a 1 uF NP capacitor coupling it to the mix.
My measurements confirmed that the low band VCA had good squarewave response before the cap, but not after it. So I added a much larger cap in parallel with the existing cap. This gave a good summation of the three bands. I touched up the unity gain trimmer for each band until I had a perfect square wave at the output of the VCA summing circuits and of equal amplitude per channel.
Listening tests verified that the timbre of an organ pedal tone remains pretty much unchanged when the 4bx is inserted into the chain now. I will test it to greater length this evening, when I can do some high volume listening and get some impressions as to how it affects kick drums and other low frequency content.
The cumulative sag of low frequency response of four devices in a series chain added up to quite a bit of total sag. I'll have to go back and re-do some of the subwoofer measurements now. It pays to have an electrically flat response when making sound level measurements with sweep tones. More on this later...